Version DFG classification

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Similar to MSC there needs to be a new version every four years. The URI scheme can be http://uri.gbv.de/terminology/dfg2020/, http://uri.gbv.de/terminology/dfg2024/ ...

This is needed to manage mappings from one version to the next with Cocoda.

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Requires gbv/jskos-proxy#31 to enable URI resolving of http://uri.gbv.de/terminology/dfg/ and other terminologies not stored in DANTE.

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The namespace of versions are being introduced by TIB in https://github.com/tibonto/DFG-Fachsystematik-Ontology as this:

• https://w3id.org/dfgfo/2020, https://w3id.org/dfgfo/2024 ... the classification
• https://w3id.org/dfgfo/2020/, https://w3id.org/dfgfo/2024/ ... URI prefixes

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The versions are in https://github.com/gbv/jskos-data/tree/master/dfg and could be imported into JSKOS server but previous versions (2016 and 2020) don't have BARTOC uri, so open question is how to use them in Cocoda. Each version requires a BARTOC URI as well but BARTOC does not support versions yet.